1. Field of the Invention
The present invention relates to a technique of establishing clock synchronization in a synchronous network such as an optical communications network.
2. Description of the Related Art
A synchronous network environment exists which requires clock synchronization to be established over the whole network.
FIG. 1 shows an exemplary configuration of a synchronous network.
Transmission units 101 are interconnected by, for example, optical transmission lines 103 that conform to the SONET (Synchronous Optical Network) standards and are partly interconnected by, for example, DS1 metal transmission lines 104. Some of the transmission units 101 are connected to an exchange 105 directly or through repeaters 106.
In such a network environment, usually a clock generator 102, called a BITS (Building Integrated Timing Supply), is provided which provides a high-precision clock reference to the transmission units 101 within the network over the DS1 metal transmission lines 104.
Recently, as overhead information (control information) sent over the optical transmission lines 103 or the metal transmission lines 104, synchronization messages have been defined which are messages that indicate quality levels of various clock references input to the transmission units 101. This requires each transmission unit to have the functions of detecting, controlling and sending these synchronization messages.
On the DS1 metal transmission line 104, the synchronization messages are sent over a DS1 ESF data link that is a control link.
FIG. 2 shows a data format for data transmission over the DSI metal transmission line 104. In this data format, one frame (125 microseconds and 193 bits) is composed of a set of 24 item slots of 8-bit data, and one multiframe is composed of a set of 24 frames. One frame is transmitted bit by bit starting with the frame bit #0 and ending with the frame bit 192, and one multiframe is transmitted starting with the frame 1 and ending with the frame 24.
FIG. 3 shows a data format for DS1 frame bits. 13 bits among the frame bits #0 to #4439 within one multiframe composed of 24 frames, which are marked with X in the field DL (Data Link) shown in FIG. 3 make up a data link having a transmission rate of 4 kb/s (kilobits per second), whereby the DS1 ESF data link is formed. In FIG. 3, the FPS field indicates a framing pattern sequence for frame extraction and the CRC field indicates cyclic redundancy check channels for data error detection and correction.
In the SONET optical transmission line 103, the synchronization messages are transmitted using an S1 byte within a line overhead that is a unit of control information.
FIG. 4 shows a data format for the overhead in SONET. Its details are beyond the scope of the present invention and hence description thereof is omitted herein. The S1 byte is placed in the byte position indicated by * in the line overhead section shown in FIG. 4 and then transmitted.
FIG. 5 shows the contents of the quality levels presented by the synchronization messages. The quality levels are defined with ppm as units. For each of the quality levels, a quality level value (1 to 7) and set values on the DS1 ESF data link and the S1 byte in the synchronization message corresponding to the quality level are indicated.
Typical examples will be described hereinafter. "Stratum 1 Traceable" indicates the highest quality level, which corresponds to the quality level of the clock reference that the BITS 102 (see FIG. 1) provides. "Stratum 3 Traceable" and ".+-.20 ppm Clock Traceable" correspond to the quality level of the internal clock of the transmission unit 101. "Stratum 4 Traceable" indicates a quality level that may occur on the DS1 metal transmission line 104. This quality level is one that rarely occurs on the SONET optical transmission line 103, and hence the S1 byte value therefor is not defined. "Don't Use for Synchronization" is a quality level for informing that no clock reference can be used for synchronous control. "Network Provider Specific Synchronization" is a quality level that is reserved for future use.
To process the synchronization messages, each transmission unit 101 of FIG. 1 has come to be required to have a function of synchronizing with the clock reference of the highest quality level, by comparing synchronization messages received together with a plurality of clock references received over transmission lines which are terminated by that transmission unit itself, a function of outputting a synchronization message corresponding to the quality level of the clock reference that is currently in use to the S1 byte that is an overhead byte on the SONET optical transmission line 103 terminated by that transmission unit, and a function of outputting the synchronization message corresponding to that quality level to the DS1 ESF data link on the DS1 metal transmission line 104 in the case where that transmission unit terminates the DS1 metal transmission line 104 as well. It is considered that such functions will be widely applied to all synchronous networks.
In the following descriptions, of clock references that are available to the transmission unit 101 for its operation or that can be output to other units as a timing supply, a clock reference that the transmission unit is actually using for its operation or outputting for timing supply is referred to as an active reference for the sake of convenience, as shown in FIGS. 6 or 7.
Conventionally, in selecting the active reference, the user uses a command to specify a group of transmission lines or BITS inputs which contain a clock reference that he or she wishes to use as the active reference, whereby the active reference is fixedly determined and selectable clock references are also fixedly determined.
As an example, when, in FIG. 8, the group that contains a clock reference that the user wishes to use as the active reference is set as group #1, the group #1 work line, the group #1 protection line, the holdover clock and the internal clock are set as the selectable clock references and the group #1 work line is set as the active reference. Here, the holdover clock refers to a clock that the transmission unit 101 outputs in synchronism with the past active reference stored by that unit when the current active reference fails. The internal clock refers to a clock which is output by the internal clock generator of the transmission unit 101 at its own timing. The work line refers to the SONET optical transmission line 103 that is normally employed. The protection line refers to a backup for the SONET optical transmission line 103.
In this example, in the event that a line failure occurs in the group #1 work line, the active reference is switched to the group #1 protection line. Moreover, in the event of a line failure in that line, the active reference is switched to the holdover clock. Furthermore, in case where a line failure occurs in that line, the active reference is switched to the internal clock. In these cases, none of the clock references associated with the group #2 can be selected.
Thus, in the conventional system, once a group is selected, clock references selectable as the active reference are decided fixedly. Also, the priority of the clock references selectable as the active reference is decided fixedly. That is, in the conventional system, clock references selectable as the active reference and the priority used in selecting among the clock references are decided fixedly by specifying a group. In the selected group, priority is established among the clock references such that the work line (primary line) clock reference, the protection line (secondary line) clock reference, the holdover clock and the internal clock are placed in the order of descending priorities.
In the conventional system, therefore, since selectable clock references and their priorities cannot be decided flexibly, the possibility exists that, even if there is a clock reference of a high quality, it may not be selected as the active reference. As a result, in the entire network, a clock reference of the highest quality is not always selected, so that the network may be synchronized with a clock of a low quality. This is a first problem with the conventional technique.
In general, some of clock references cause a state called a timing loop, where, in a specific area within the network, a loop is formed in which only clocks of the same quality are referenced and clocks of higher quality cannot be referenced. Such clock references are those that should not be selected. In the conventional system in which a group the user wishes to use for the active reference is selected, there is the possibility that a clock reference that may cause such a timing loop as described above is accidentally selected as the active reference, and consequently that clock reference produces a timing loop. This is a second problem with the conventional technique.
Moreover, in the conventional system, restrictions are made on clock references selectable as the active reference; sufficient redundancy does not exist in selectable clock references. Thus, there frequently occurs a phenomenon in which the active reference is set to either the holdover clock or the internal clock and the whole network is synchronized with a clock of low quality. This is a third problem with the conventional technique.
Furthermore, in the conventional system in which a group is specified for use as the active reference, even when the transmission unit 101 outputs a clock reference as a timing supply output to another unit, the primary clock in the timing supply output is derived from the work line in the specified group and the secondary clock is derived from the protection line in the specified group, as shown in FIG. 9.
For this reason, even if a clock reference of higher quality exists in another group, it is impossible to select it as the active reference and output it as the timing supply output to another unit. This is a fourth problem with the conventional technique.
In the conventional system, in the event that the active reference fails, an alarm indication signal (AIS) may be immediately sent as the timing supply output. That is, there is little redundancy in clock references as the timing supply output. This is a fifth problem with the conventional technique.
Furthermore, conventionally it is impossible to send clocks of the highest quality all the time for both the primary and secondary clocks in the timing supply output. This is a sixth problem with the conventional technique.
Furthermore, conventionally it is possible to use, in combination with the active reference selection described above, an active reference forced switching command that, once the active reference is switched to a clock reference, does not allow the active reference to be switched to another clock reference. With the use of this command, it becomes possible to switch the active reference to any clock reference by force. In the conventional system, therefore, in the event that the active reference is switched to a failing clock reference by force or a clock reference to which the active reference has been switched fails, subsequent clock selection may become impossible. In this case, the system would fail. This is a seventh problem with the conventional system.
In addition, in the conventional system, when the active reference is controlled so that a clock reference of high priority will be selected all the time on the basis of synchronization messages, there arises the possibility that the following phenomenon may occur. Assume, for example, that clock references selectable as the active reference include the group #1 work line, the group #1 protection line, the holdover clock, and the internal clock, they are selected by priority in the order in which they are listed, and the clock reference of the highest quality is the group #1 protection line. In this case, the group #1 protection line is selected as the active reference conventionally. Next, when the group #1 work line becomes equal in quality to the group #1 protection line, the active reference is switched to the group #1 work line conventionally. Thus, if a clock reference that is higher in priority than a clock reference which is currently selected as the active reference becomes equal in quality to the active reference, then the active reference will be switched to the clock reference of higher priority at all times. Thus, switching may occur frequently, increasing the burden of processing on the CPU in the transmission unit 101. This is an eighth problem with the conventional technique.